The genus Morbillivirus is one of the genera under the family Paramyxoviridae of the order Mononegavirales, including many pathogenic viruses such as the measles virus that causes “measles”—an acute eruptive disorder. The measles virus widely infects infants, expressing symptoms such as fever, eruptions, cough, and such, occasionally causing severe complications such as measles-associated encephalitis, pneumonia, and such, sometimes even death. Furthermore, though very rarely, the measles virus sustains its infection even after the cure of infectious symptoms, causing encephalitis with a poor prognosis, named subacute sclerosing panencephalitis (SSPE). The one and only effective prophylactic means is vaccination with an attenuated measles virus vaccine.
The AIK-C strain, one of the attenuated measles virus vaccines, is a viral strain obtained by continual passage of the measles virus Edmonston strain in sheep kidney cells and chicken embryos cells. The AIK-C strain is excellent in its seroconversion rate as well as safety, which has earned it a high reputation internationally. With the spread of this attenuated measles vaccine, patients who contract measles have noticeably declined in number. In general, the seroconversion rate and safety of a vaccine are two incompatible characteristics, making it difficult to maintain both at a high standard. Therefore, if the mechanism of attenuation used in the AIK-C strain can be applied to other strains and viruses, it will be useful in the development of attenuated vaccines.
The AIK-C strain is known of its temperature-sensitivity (ts) to proliferate well at 32° C. but very poorly at 39° C. (Sasaki, K., Studies on the modification of the live AIK measles vaccine. I. Adaptation of the further attenuated AIK measles virus (the strain AIK-L33) to chick embryo cells. Kitasato Arch. of Exp. Med., 47, 1–12, 1974). However, the mechanism by which this virus strain becomes temperature-sensitive remains unclear.